Wastewater Management

Introduction
Wastewater is water whose physical, chemical or biological properties have
been changed as a result of the introduction of certain substances which render
it unsafe for some purposes such as drinking. The day-to-day activities of man
is mainly water dependent and therefore discharge ‘waste’ into water. Some of
the substances include body wastes (feces and urine), hair shampoo, hair, food
scraps, fat, laundry powder, fabric conditioners, toilet paper, chemicals,
detergent, household cleaners, dirt, micro-organisms (germs) which can make
people ill and damage the environment. It is known that much of water
supplied ends up as wastewater which makes its treatment very important.
Wastewater treatment is the process and technology that is used to remove
most of the contaminants that are found in wastewater to ensure a sound
environment and good public health. Wastewater Management therefore
means handling wastewater to protect the environment to ensure public
health, economic, social and political soundness (Metcalf and Eddy, 1991).

History of wastewater treatment

Wastewater treatment is a fairly new practice although drainage systems were
built long before the nineteenth century. Before this time, “night soil” was
placed in buckets along streets and workers emptied them into “honeywagon”
tanks. This was sent to rural areas and disposed off over agricultural lands. In
the nineteenth century, flush toilets led to an increase in the volume of waste
for these agricultural lands. Due to this transporting challenge, cities began to
use drainage and storm sewers to convey wastewater into waterbodies against
the recommendation of Edwin Chadwick in 1842 that “rain to the river and
sewage to the soil”. The discharge of waste into water courses led to gross
pollution and health problems for downstream users.
In 1842, an English engineer named Lindley built the first “modern” sewerage
system for wastewater carriage in Hamburg, Germany. The improvement of the
Lindley system is basically in improved materials and the inclusion of
manholes and sewer appurtenances— the Lindley principles are still upheld
today. Treatment of wastewater became apparent only after the assimilative
capacity of the waterbodies was exceeded and health problems became
intolerable. Between the late 1800s and early 1900s, various options were tried
until in 1920, the processes we have today were tried. Its design was however
empirical until midcentury. Centralized wastewater systems were designed and
encouraged. The cost of wastewater treatment is borne by communities
discharging into the plant.
Today there have been great advances to make portable water from
wastewater. In recent times, regardless of the capacity of the receiving stream,
a minimum treatment level is required before discharge permits are granted
(Peavy, Rowe and Tchobanoglous, 1985). Also presently, the focus is shifting
from centralized systems to more sustainable decentralized wastewater
treatment (DEWATS) especially for developing countries where wastewater
infrastructure is poor and conventional methods are difficult to manage (Adu-
Ahyia and Anku, 2010).

Objectives of wastewater treatment

Wastewater treatment is very necessary for the above-mentioned reasons.
It is more vital for the:
Reduction of biodegradable organic substances in the environment:
organic substances such as carbon, nitrogen, phosphorus, Sulphur in
organic matter needs to be broken down by oxidation into gases which is
either released or remains in solution.
Reduction of nutrient concentration in the environment: nutrients such
as nitrogen and phosphorous from wastewater in the environment enrich
water bodies or render it
eutrophic leading to the growth of algae and other aquatic plants. These
plants deplete oxygen in water bodies and this hampers aquatic life.
Elimination of pathogens: organisms that cause disease in plants,
animals and humans are called pathogens. They are also known as
micro-organisms because they are very small to be seen with the naked
eye. Examples of micro-organisms include bacteria (e.g., vibro cholerae),
viruses (e.g. enterovirus, hepatits A & E virus), fungi (e.g. candida
albicans), protozoa (e.g entamoeba hystolitica, giardia lamblia) and
helminthes (e.g. schistosoma mansoni, asaris lumbricoides). These micro-
organisms are excreted in large quantities in faeces of infected animals
and humans (Awuah and Amankwaa-Kuffuor, 2002).
Recycling and Reuse of water: Water is a scarce and finite resource which
is often taken for granted. In the last half of the 20th century, population
has increased resulting in pressure
on the already scarce water resources. Urbanization has also changed
the agrarian nature of many areas. Population increase means more food
has to be cultivated for the growing population and agriculture as we
know is by far the largest user of available water which means that
economic growth is placing new demands on available water supplies.
The temporal and spatial distribution of water is also a major challenge
with groundwater resources being overdrawn (National Academy,
2005). It is for these reasons that recycling and reuse is crucial for
sustainability.

Types of wastewater
Wastewater can be described as in the figure below.
 Wastewater

Stormwater Runoff Industrial Domestic

Grey water Blackwater

Bathroom Laundry Kitchen Urine Faeces

Definition of concepts and terminology
Stormwater Runoff is water from streets, open yard etc. after a rainfall event
which run through drains or sewers.
Industrial wastewater is liquid waste from industrial establishments such as
factories, production units etc.
Domestic wastewater also known as municipal wastewater is basically
wastewater from residences (homes), business buildings (e.g. hotels) and
institutions (e.g. university). It can be categorized into greywater and
blackwater.
Greywater also known as sullage is liquid waste from washrooms,
laundries, kitchens
which does not contain human or animal excreta.
Blackwater is wastewater generated in toilets. Blackwater may also contain
some flush water besides urine and faeces (excreta). Urine and faeces together
is sometimes referred to as night soil.
Sewage is the term used for blackwater if it ends up in a sewerage system.
Septage is the term used for blackwater if it ends up in a septic tank.
Sewerage system is the arrangement of pipes laid for conveying sewage.
Influent is wastewater which is yet to enter in a wastewater treatment plant
or liquid waste that is yet to undergo a unit process or operation.
Effluent is the liquid stream which is discharged from a wastewater treatment
plant or discharge from a unit process or operation.
Sludge is the semi-solid slurry from a wastewater treatment plant.
On-Site System: this is wastewater disposal method which takes place at the
point of waste production like within individual houses without transportation.
On- site methods include dry methods (pit latrines, composting toilets), water
saving methods (pour- flush latrine and aqua privy with soakage pits and
methods with high water rise (flush toilet with septic tanks and soakage pit,
which are not emptied).
Off-Site System: in this system, wastewater is transported to a place either
than the point of
production. Off- site methods are bucket latrines, pour-flush toilets with
vault and tanker removal and conventional sewerage system.
Conventional sewerage systems can be combined sewers (where wastewater
is carried with
storm water) or separated sewers.
Septic Tank is an on-site system designed to hold blackwater for
sufficiently long period to allow sedimentation. It is usually a water tight
single storage tank.
Faecal sludge refers to all sludge collected and transported from on-site
sanitation systems by vacuum trucks for disposal or treatment.
Unit Operation: this involves removal of contaminants by physical forces.
Unit Process: this involves biological and/or chemical removal of
contaminants.
Wastewater Treatment Plant is a plant with a series of designed unit
operations and processes that aims at reducing certain constituents of
wastewater to acceptable levels.

Characteristics of wastewater
Depending on its source, wastewater has peculiar characteristics. Industrial
wastewater with characteristics of municipal or domestic wastewater can be
discharged together. Industrial wastewater may require some pretreatment if
it has to be discharged with domestic wastewater. The characteristics of
wastewater vary from industry to industry and
therefore, would have different treatment processes—for example a cocoa
processing company may have a skimming tank in its preliminary treatment
stage to handle for instance spilt cocoa butter while a beverage plant may
skip this in the design. In general, the contaminants in wastewater are
categorized into physical, chemical and biological. Some indicator measured
to ascertain these contaminants include (Peavy, Rowe and Tchobanoglous,
1985 & Obuobie et al., 2006):

Physical
Electrical Conductivity (EC) indicates the salt content
Total Dissolved Solids (TDS) comprise inorganic salts and small amounts
of organic matter dissolved in water
Suspended solids (SS) comprise of solid particles suspended (but not
dissolved) in water
Chemical
 Dissolved Oxygen (DO) indicates the amount of oxygen in water
 Biochemical oxygen demand (BOD) indicates the amount of
oxygen required by aerobic microorganisms to decompose the
organic matter in a sample of water in a defined time period.
 Chemical oxygen demand (COD) indicates the oxygen equivalent of
the organic matter content of a sample that is susceptible to oxidation
by a strong chemical oxidant
Total Organic Compound (TOC)
NH4-N and NO3-N show dissolved nitrogen (Ammonium and Nitrate,
respectively).
Total Kjeldhal Nitrogen is a measurement of organically-bound ammonia
nitrogen.
Total-P reflects the amount of all forms of phosphorous in a sample.
Biological
Total coliforms (TC) are encompassing faecal coliforms as well as
common soil microorganisms, and is a broad indicator of possible water
contamination.
Faecal coliforms (FC) is an indicator of water contamination with faecal
matter. The common lead indicator is the bacteria Escherichia coli or E. coli.
Helminth analysis looks for worm eggs in the water

Process of wastewater treatment

Due to the nature of contaminants in wastewater—physical, chemical and
biological, the unit operations and processes in wastewater treatment can
also be categorized as such. The units operations and processes in Waste-
water treatment are summarized as follows (Economic and Social
Commission for Western Asia (ESCWA), 2003):
Physical unit operations
Screening
Comminution
Flow equalization
Sedimentation
Flotation
Granular-medium filtration
Chemical unit operations
Chemical precipitation
Adsorption
Disinfection
Dichlorination
Other chemical applications
Biological unit operations
Activated sludge process
Aerated lagoon
Trickling filters
Rotating biological contactors
Pond stabilization
Anaerobic digestion

Levels of wastewater treatment

There are three broad levels of treatment: primary, secondary and tertiary.
Sometimes, preliminary treatment precedes primary treatment.
Preliminary treatment: removes coarse suspended and grits. These can
be removed by
screening, and grit chambers respectively. This enhances the operation and
maintenance of subsequent treatment units. Flow measurement devices,
often standing-wave flumes, are necessary at this treatment stage (FAO,
2006).
Primary treatment removes settleable organic and inorganic solids by
sedimentation and
floating materials (scum) by skimming. Up to 50% of BOD5, 70% of
suspended solids and 65% of grease and oil can be removed at this stage.
Some organic nitrogen, organic phosphorus, and heavy metals are also
removed. Colloidal and dissolved constituents are however not removed at
this stage. The effluent from primary sedimentation units is referred to as
primary effluent (FAO, 2006).
Secondary treatment is the further treatment of primary effluent to
remove residual
organics and suspended solids. Also, biodegradable dissolved and colloidal
organic matter is removed using aerobic biological treatment processes.
The removal of organic matter is when nitrogen compounds and
phosphorus compounds and pathogenic microorganisms are removed. The
treatment can be done mechanically like in trickling filters, activated sludge
methods rotating biological contactors (RBC) or non-mechanically like in
anaerobic treatment, oxidation ditches, stabilization ponds etc.
Tertiary treatment or advance treatment is employed when specific
wastewater constituents
which cannot be removed by secondary treatment must be removed.
Advance treatment removes significant amounts of nitrogen, phosphorus,
heavy metals, biodegradable organics, bacteria and viruses. Two methods
can be used effectively to filter secondary effluent—traditional sand (or
similar media) filter and the newer membrane materials. Some filters have
been improved, and both filters and membranes also remove helminths. The
latest method is disk filtration which utilizes large disks of cloth media
attached to rotating drums for filtration (FAO, 2006).
At this stage, disinfection by the injection of Chlorine, Ozone and Ultra Violet
(UV) irradiation can be done to make water meet current international
standards for agricultural and urban re-use.

Methods of wastewater treatment

There are conventional and non-conventional wastewater treatment
methods which have been proven and found to be efficient in the treatment
of wastewater. Conventional methods compared to non-conventional
wastewater treatment methods has a relatively high
level of automation. Usually have pumping and power requirements. They
require skilled labor for operation and maintenance of the system
Source: NPTEL (accessed 2010)
Fig. 2. Typical Wastewater Treatment Plant

Conventional methods
Examples of conventional wastewater treatment methods include
activated sludge, trickling filter, rotating biological contactor methods.
Trickling filters and Rotating Biological Contactors are temperature
sensitive, remove less BOD, and trickling filters cost more to build than
activated sludge systems. Activated sludge systems are much more
expensive to operate because energy is needed to run pumps and
blowers (National Programme on Technology Enhanced Learning
(NPTEL), 2010).
These methods are discussed in detail in the subsequent sections.

Activated sludge
Activated sludge refers to biological treatment processes that use a
suspended growth of organisms to remove BOD and suspended solids. It
is based on the principle that intense wastewater aeration to forms flocs
of bacteria (activated sludge), which degrade organic matter and be
separated by sedimentation. The system consists of aeration and settling
tanks with other appurtenances such as return and waste pumps, mixers
and blowers for aeration and a flow measurement device. To maintain
the concentration of active bacteria in the tank, part of the activated
sludge is recycled.
Primary effluent (or plant influent) is mixed with return activated sludge
to form mixed liquor which is aerated for a specified length of time. By
aerating the system, activated sludge organisms use the available
organic matter as food, thereby, producing stable solids and more
organisms. The suspended solids produced by the process and the
additional organisms become part of the activated sludge. The solids
are then separated from the
wastewater in the settling tank and are returned to the influent of the
aeration tank (return activated sludge). Periodically the excess solids and
organisms are removed from the system (waste activated sludge) to
enhance the performance of the system.
Factors such as temperature, return rates, amount of oxygen available,
amount of organic matter available, pH, waste rates, aeration time, and
wastewater toxicity affect the performance of an activated sludge
treatment system. A balance therefore must be maintained between the
amount of food (organic matter), organisms (activated sludge) and
dissolved oxygen (NPTEL, 2010).
Activated Sludge systems are requiring less space compared to trickling
filter and has high effluent quality. The disadvantage is that BOD is
higher at one end of the tank than the other the microorganisms will be
physiologically more active at that end than the other unless a complete
Filter material Distributor

Filter Floor

Underdrain
mixing activated sludge system process is used.

Source: Mountain Empire College, 2010

Fig. 3. An activated Sludge System

Trickling filter:
It is a growth process in which microorganisms responsible for treatment
are attached to an inert packing material. It is made up of a round tank filled
with a carrier material (volcanic rock, gravel or synthetic material).
Wastewater is supplied from above and trickles through filter media
allowing organic material in the wastewater to be adsorbed by a population
of microorganisms (aerobic, anaerobic, and facultative bacteria; fungi; algae;
and protozoa) attached to the medium as a biological film or slime layer
(approximately 0.1 to 0.2 mm thick).
Degradation of organic material by the aerobic microorganisms in the outer
part of the slime layer occurs. As the layer thickens through microbial
growth, oxygen cannot penetrate the medium face, and anaerobic organisms
develop. The biological film continues to grow to such a point that
microorganisms near the surface cannot cling to the medium, and a portion
of the slime layer falls off the filter. This process is known as sloughing. The
sloughed solids are picked up by the underdrain system and transported to
a clarifier for removal from the wastewater (US EPA, 2000).

Trickling filters are efficient in that effluent quality in terms of BOD and
suspended solids removal is high. Its operational costs are relatively low
due to low electricity requirements. The process is simpler compared to
activated sludge process or some package treatment plants. Its operation
and maintenance requirements is however high due to the use of electrical
power. Skilled labor is required to keep the trickling filter running trouble-
free:
e.g., prevent clogging, ensure adequate flushing, control filter flies. It is
suitable for some relatively wealthy, densely populated areas which have a
sewerage system and centralized wastewater treatment; also suitable for
greywater treatment.
It also requires more space compared to some other technologies and has
potential for odour and filter flies (NPTEL, 2010).

Distributor
Filter material

Filter Floor
Underdrain

Source: ESCWA, 2003

Fig. 4. Cross section of a trickling filter

Rotating biological contactors

Rotating biological contactors (RBCs) consist of vertically arranged, plastic
media on a horizontal, rotating shaft. The plastics range from 2 – 4 m in
diameter and up to 10 mm thick (Peavy, Rowe ad Tchobanoglous, 1985).
The biomass-coated media are alternately exposed to wastewater and
atmospheric oxygen as the shaft slowly rotates at 1–1.5 rpm (necessary to
provide hydraulic shear for sloughing and to maintain turbulence to keep
solid in suspension), with about 40% of the media submerged. High surface
area allows a large, stable biomass population to develop, with excess
growth continuously and automatically shed and removed in a downstream
clarifier. Thickness of biofilm may reach 2 – 4 mm depending on the
strength of wastewater and the rotational speed of the disk.
RBC systems are relatively new, though it appeared to be best suited to treat
municipal wastewater (Peavy, Rowe ad Tchobanoglous, 1985), they have
been installed in many petroleum facilities because of their ability to quickly
recover from upset conditions (Schultz, 2005). The RBC system is easily
expandable should the need arise, and RBCs are also very easy to enclose
should volatile organic content containment become necessary. RBCs have
relatively low power requirements and can even be powered by
compressed air which can also aerate the system. They follow simple
operating procedures and thus require a moderately skilled labour. RBCs
are however capital intensive to install and sensitive to temperature

Primary treatment Secondary Clarifier

Influent
Effluent

Solids Removal
Source: ESCWA, 2003
Fig. 5. Rotating Biological
Contactors
Membrane bioreactors
This method performs more than just one treatment step. Membrane
bioreactor (MBR) systems are unique processes, which combine anoxic-
and aerobic-biological treatment with an integrated membrane system that
can be used with most suspended-growth, biological wastewater-treatment
systems.

Source: Google Images

Fig. 6. Membrane Bioreactor

Wastewater is screened before entering the biological treatment tank.

Aeration within the aerobic-reactor zone provides oxygen for biological
respiration and maintains solids in suspension. MBR relies on
submerged membranes to retain active biomass in the process. This
allows the biological process to operate at longer than normal sludge
ages (typically 20- 100 days for a MBR) and to increase mixed-liquor,
suspended-solids (MLSS) concentrations (typically 8,000-15,000 mg/l)
for more effective removal of pollutants. High MLSS concentrations
reduce biological-volume requirements and the associated space needed
to only 20–30% of conventional biological processes.
MBRs cover a small land area as it eliminates the need for secondary
clarifiers, which equates to a huge savings in both footprint and concrete
costs. They can operate at higher biomass concentrations (MLSS) than
conventional treatment processes. Facility can be expanded by simply
adding more membranes to existing basins without expanding land cover.
For reuse quality, it does not require tertiary treatment, polymer addition,
or any further treatment processes to meet standards. This reduction in
the number of unit processes further improves system reliability and
reduces operation activities (TEC, 2010). The generally high effluent
quality reduces the burden on disinfection in the treatment process.

Non-conventional methods
These are low-cost, low-technology, less sophisticated in operation and
maintenance biological treatment systems for municipal wastewater.
Although these systems are land intensive by comparison with the
conventional high-rate biological processes, they are often more
effective in removing pathogens and do so reliably and continuously if
system is properly designed and not overloaded (FAO, 2006). Some of
the non-conventional methods include stabilization ponds, constructed
wetlands, oxidation ditch, soil aquifer treatment.

Waste stabilization ponds

Waste Stabilization Ponds are man-made, shallow basins which
comprise of a single series or several series of anaerobic, facultative or
maturation ponds. This is a low-technology treatment process with 4 or
5 pounds of different depths with different biological activities.
Treatment of the wastewater occurs as constituents are removed by
sedimentation or transformed by biological and chemical processes
(National Academy, 2005). The anaerobic ponds are mainly designed for
the settling and removal of suspended solids as well as the breakdown of
some organic matter (BOD 5). In facultative ponds, organic matter is
further broken down to carbon dioxide, nitrogen and phosphorous by
using oxygen produced by algae in the pond. Maturation ponds usually
remove nutrients and pathogenic micro- organisms, thus primary
treatment occurs in anaerobic ponds while secondary and tertiary
treatment occurs in facultative and maturation ponds respectively
(Awuah, 2002).
Anaerobic ponds are usually between 2-5 m deep and receive high
organic loads equivalent to 100g BOD5 and m3/d leading to anaerobic
conditions throughout the pond (Mara et al., 1992). If properly designed,
anaerobic ponds can remove 60% of BOD5 at 200 C. Facultative ponds
are 1-2 m deep and usually receive the effluent from an anaerobic pond.
In some designs, they receive raw wastewater acting as primary
facultative pond. In facultative ponds organic loads are lower and allows
for algal growth which accounts for the dark green colour of wastewater.
Algae and aerobic bacteria generate oxygen which breaks down BOD5.
Good wind velocity generates mixing of wastewater in ponds thus
leading to uniform mixing of BOD5, oxygen, bacteria and algae which
better stabilizes waste. Maturation ponds are usually shallow ponds of
about 1.0-1.5 m deep allowing aerobic conditions in for the treatment of
facultative pond effluents. Further reduction of organic matter, nutrients
and pathogenic microorganisms occurs here. Algal population in
maturation ponds is more diverse and removal of nitrogen and
ammonia is more prominent. It usually flows under gravity from one
pond to the other and mostly does not require any pumping. It is less
energy dependent thus plant activities cannot be interrupted due to
power cuts. Its disadvantages however include odour problems and it
requires a large area of land to function properly. The figure below
shows some possible combinations.

Constructed wetlands
Constructed Wetlands (CW’s) are planned systems which are designed and
constructed to employ wetland vegetation to assist in treating wastewater
in a more controlled environment than occurs in natural wetlands
(Kayombo et al., 2000). They are an eco- friendly and a suitable
alternative for secondary and tertiary treatment of municipal and industrial
wastewater. They are suitable for the removal of organic materials,
suspended solids, nutrients, pathogens, heavy metals and toxic pollutants.
They are not ideal for the treatment of raw sewage, pre-treatment of
industrial wastewater to maintain the biological balance of the wetland
ecosystem.
There are two types of CW’s namely Free Water Surface (FWS) and
Subsurface Flow (SSF) systems. As the name suggests, with FWS, water
flows above the ground and plants are rooted in the sediment layer
below the water column. With SSF, water flows through a porous media
such as gravels in which the plants are rooted. From a public health
perspective, SSF should be used in primary treatment of wastewater
because there is no direct contact of wastewater with atmosphere.

Source: ESCWA, 2003

Fig. 8. Free Water Surface System
The SSF is mostly anoxic or anaerobic as oxygen supplied by the roots of
plants is used up in biofilm growth and as such does not reach the water
column. The flow of water in SSF can be horizontal or vertical (Kayombo
et al., 2000). FWS are suitable for treating secondary and tertiary
effluents and also providing habitat due to aerobic conditions at and
near the surface of the water. The condition at the bottom sediment is
however anoxic. Wetlands plants or macrophytes utilized in CW’s include
Cattails (Typha latifolia sp), Scirpus (Bulrus), Lemna (duckweed), Eichornia

crassipes (water hyacinth), Pistia stratiotes (water lettuce) Hydrocotyle spp.

(pennywort), Phragmites (reed) have been known and used in constructed
wetlands.
Source: ESCWA, 2003
 Sub-surface flow system
CWs are relatively cheaper to construct operate and easy to maintain.
This is an important decision variable for developing countries. In Egypt,
according to Hendy (2006), between 2000 and 2004, a 60-acre artificial
wetland constructed cost 25% the cost of conventional sewage
treatment plant. They provide effective and reliable treatment of
wastewater and are tolerant to fluctuating hydrologic and contaminant
loading rates. With the example in Egypt, $9 million (US) was spent to
treat an initial volume of 25,000 metric tons per day. After a year of use,
it was determined that the wetland was capable of treating 40,000
metric tons per day (Hendy, 2006). Also, a study conducted by
Ratnapriya et al., (2009) revealed over 60% removal of BOD5, COD,
nitrogen among others.
CWs also provide indirect benefits such as enjoying the scenic views of
green spaces, encouraging wildlife habitats and providing recreational
and educational centres. Again, in Egypt, the fishing industry is
expanding since the wastewater was no longer being discharged directly
into the waterways, the local fisheries improved. According to Hendy
(2006), nitrates and heavy metals were filtered out, leaving the fish
healthier, larger and in abundant quantity. This indirectly led to poverty
reduction.
They however have some disadvantages such as land requirements, its
design and operation criteria is presently imprecise. CWs are
biologically and hydrologically complex and its process dynamics are
not completely understood. Sometimes there are cost implications of
gravels fills and site grading during construction (Kayombo et al., 2000).
It must be emphasized that if properly designed, constructed wetlands
should not breed pests and mosquitoes.

Oxidation ditches
An oxidation ditch is a modified activated sludge biological treatment
process that utilizes hydraulic retention time of 24 - 48 hours, and a
sludge age of 12 - 20 days. to remove biodegradable organics. Oxidation
ditches are typically complete mix systems, but can be modified. Typical
oxidation ditch treatment systems consist of a single or multichannel
configuration within a ring, or oval. Preliminary treatment, such as bar
screens and grit removal, normally precedes the oxidation ditch.
Primary settling prior to an oxidation ditch is sometimes practiced and
tertiary filters may be required after clarification, depending on the
effluent requirements. Disinfection is required and reaeration may be
necessary prior to final discharge. Horizontally or vertically mounted
aerators provide circulation, oxygen transfer, and aeration in the ditch.
Flow to the oxidation ditch is aerated and mixed with return sludge from
a secondary clarifier. The mixing process entrains oxygen into the mixed
liquor to foster microbial growth and the motive velocity ensures
contact of microorganisms with the influent. Aeration increases
dissolved oxygen concentration but decreases as biomass takes up
oxygen during mixing in the ditch. Solids also remain in suspension
during circulation (USEPA, 2000).
They require more power than waste stabilization ponds less land, and
are easier to control than processes such as activated sludge process. A
typical process flow diagram of treatment plant using an oxidation ditch
is shown in Figure 10.
 (UASB)
Up flow anaerobic sludge blanket
Up flow anaerobic sludge blanket is an anaerobic process using blanket
of bacteria (see Figure 11) to absorb polluting load. It is a form of
anaerobic digester which forms a blanket of granular sludge which
suspends in the tank. Wastewater flows upwards through the blanket
and is processed (degraded) by the anaerobic microorganisms. The
upward flow combined with the settling action of gravity suspends the
blanket with the aid of flocculants. Small sludge granules begin to form
whose surface area is covered in aggregations of bacteria. In the absence
of any support matrix, the flow conditions create a selective
environment in which only those microorganisms, capable of attaching
to each other, survive and proliferate.

Gas/Solid Separator

Effluent

Sludge

Influent Sludge bed

Fig. 11. Up flow Anaerobic Sludge Blanket

Eventually the aggregates form into dense compact biofilms referred to
as granules. The UASB reactor works best when desirable micro-
organisms are retained as highly active and fast settling granules. In the
UASB reactor, when high solids retention time is met, separation of gas,
sludge solids from the liquid occurs. The special Gas-Solid-Liquid
Separators in the reactor enable collection of biogases and recycle of
anaerobic biomass. Biogas contains 50 to 80% methane. UASB is suitable
for the primary treatment of high-COD mainly soluble industrial
effluents. It can also be used for the treatment of wastewater effluents of
low and medium strength. It is suited to hot climates Low energy
requirement, less operation and maintenance, lower skill requirement
for operation, less sludge production, resource recovery through biogas
generation and stabilized waste as manure. UASBs however have
relatively poor effluent quality than processes such as activated sludge
process (Tare and Nema, 2010). The technology however, needs
constant monitoring to ensure that the sludge blanket is maintained, and
not washed out. The heat produced as a by-product of electricity
generation can be reused to heat the digestion tanks.

Soil aquifer treatment

Soil matrix has quite a high capacity for treatment of normal domestic
sewage, as long as capacity is not exceeded. Partially-treated sewage
effluent is allowed to infiltrate in controlled conditions to the soil. The
unsaturated or "vadose" zone then acts as a natural filter and can
remove essentially all suspended solids, biodegradable materials,
bacteria, viruses, and other microorganisms. Significant reductions in
nitrogen, phosphorus, and heavy metals concentrations can also be
achieved. After the sewage, treated in passage through the vadose zone,
has reached the groundwater it is usually allowed to flow some distance
through the aquifer for further purification before it is collected through
the aquifer. Soil-aquifer treatment is a low-technology, advanced
wastewater treatment system. It also has an aesthetic advantage over
conventionally treated sewage since effluent from an SAT system is clear
and odour-free and it is viewed as groundwater either than effluent.
Discharge effluent should travel sufficient distance through the system
and residence times should be long enough, to produce effluent of
desired quality (FAO, 2006).

Faecal sludge treatment and disposal

Sewage sludge contains organic and inorganic solids that were found in
the raw wastewater. Sludge from primary and secondary clarifier as
well as from secondary biological treatment need to be treated. The
generated sludge is usually in the form of a liquid or semisolid,
containing 0.25 to 12 per cent solids by weight, depending on the
treatment operations and processes used. Sludge is treated by means of
a variety of processes that can be used in various combinations.
Thickening, conditioning, dewatering and drying are primarily used to
remove moisture from sludge, while digestion, composting, incineration,
wet-air oxidation and vertical tube reactors are used to treat or stabilize
the organic material in the sludge (ESCWA, 2003).
Thickening: Thickening is done to increase the solids content of sludge
by the reduction of the liquid content. An increase in solids content from
3 to about 6 per cent can decrease total sludge volume significantly by
50 per cent. Sludge thickening methods are usually physical in nature:
they include gravity settling, flotation, centrifugation and gravity belts.

Stabilization: Sludge stabilization is aimed at reducing the pathogen

content, eliminate offensive odours, and reduce or eliminate the
potential for putrefaction. Some methods used for sludge stabilization
include lime stabilization, heat treatment, anaerobic digestion, aerobic
digestion and composting (ESCWA, 2003).

Wastewater reuse in agriculture

Irrigation with wastewater is both disposal and utilization and indeed is
an effective form of wastewater disposal (as in slow-rate land
treatment). However, some degree of treatment must normally be
provided to raw municipal wastewater before it can be used for
agricultural or landscape irrigation or for aquaculture.
In many industrialized countries, primary treatment is the minimum
level of preapplication treatment required for wastewater irrigation. It
may be considered sufficient treatment if the wastewater is used to
irrigate crops that are not consumed by humans or to irrigate orchards,
vineyards, and some processed food crops (FAO, 2006).
Nutrients in municipal wastewater and treated effluents are a particular
advantage as supplemental fertilizers. Success in using treated
wastewater for crop production will largely depend on adopting
appropriate strategies aimed at optimizing crop yields and quality,
maintaining soil productivity and safeguarding the environment. Several
alternatives are available and a combination of these alternatives will
offer an optimum solution for a given set of conditions. The user should
have prior information on effluent supply and its quality. Wastewater
effluent can be blended with conventional water or solely used.
Countries must develop standards in congruence with the WHO
guidelines and enforce it.

Industrial wastewater treatment

In general, the type of plant to be installed depends on the
characteristics of the wastewater produced from that industry. The basic
principle according to Kamala and Kanth Rao (1989) however is waste
prevention by good housekeeping practices that will ultimately result in
volume reduction and strength reduction. Industrial wastewater is
treated the same way as domestic or municipal sewage—preliminary,
primary, secondary and advanced treatment levels. Most of the
treatment methods discussed is also applicable. There could however be
peculiarities with different industrial depending on their major
contaminant e.g., heavy metals, dye, etc.
Industrial wastewater is generated from breweries, distilleries, textile,
chemical & pharmaceuticals and institutions and hotels, mining
activities are predominant and the major polluter of our rivers.

Challenges of wastewater management

Wastewater management though not technically difficult can sometimes
be faced with socio-economic challenges. A few of the challenges are
discussed below.

Infrastructure
Most often than not, wastewater from infrastructure are not the priority of
most politicians and therefore very little investment are made. It is however
important to consider wastewater infrastructure as equally important as
water treatment plant because almost all the water produced ends up as
wastewater.

Pollution of water sources

Effects of wastewater effluent on receiving water quality is enormous, it
changes the aquatic environment thus interrupts with the aquatic
ecosystem. The food we eat contains carbonaceous matter, nutrients, trace
elements and salts and are contained in urine and faeces (black water).
Medications (drugs), chemicals and in recent times hormones
(contraceptives) are also discharged into the wastewater treatment plant.
Discharge guidelines must be strictly adhered to. This will ensure
sustainability of water sources for posterity.

The precautionary and the polluter-pays principles which prevent or

reduce pollution to the wastewater have proven to be very efficient in
the industrialized countries and should be adapted in developing
countries as well.

Choice of appropriate technology

Because the economy of most developing countries is donor driven, funds
for wastewater plants are mainly from donors. For this reason, they tend to
propose the technology which should be adopted. For this reason, when the
beneficiaries, take over the facility, its management of the operations and
maintenance of parts become quite challenging as the technical expertise,
power requirements etc. are not sustainable.

Sludge production
Treatment of wastewater results in the production of sewage sludge.
There must be a reliable disposal method. If it must be used in
agriculture, then the risks involved must be taken into consideration.
Due to the presence of heavy metals in wastewater, it is sometimes
feared that agricultural use may lead to accumulation of heavy metals in
soils thereby contaminating of yields.

Reuse
Effluents which meet discharge standards could be used for agricultural
purposes such as aquaculture or for irrigation of farmlands. The
challenge however is that if wastewater treatment plants are not
managed and continuously monitored to ensure good effluent quality,
reuse becomes risky.

Conclusion
Wastewater is and will always be with us because we cannot survive
without water. When water supplied is used for the numerous human
activities, it becomes contaminated or its characteristics is changed and
therefore become wastewater. Wastewater can and must be treated to
ensure a safe environment and foster public health. There are
conventional and non-conventional methods of wastewater treatment
and the choice of a particular method should be based on factors such as
characteristics of wastewater whether it from a municipality or industry
(chemical, textile, pharmaceutical etc.), technical expertise for operation
and maintenance, cost implications, power requirements among others.
In most developing countries low-cost, low-technology methods such as
waste stabilization ponds have been successful whilst conventional
methods like trickling filters and activated sludge systems have broken
down. Effluent which meets set discharge standards can be
appropriately used for aquaculture and also irrigation. Though there are
a few challenges in waste water management, they can be surmounted if
attention and the necessary financial support is given to it.